1. Field of the Invention
The invention relates to an electrically conductive composite, and in particular to an electrically conductive composite serving as a bipolar plate of a fuel cell.
2. Description of the Related Art
As shown in FIG. 1, a proton exchange membrane fuel cell (PEMFC) comprises a proton exchange membrane 11 interposed between catalyst layers 13, bipolar plates 17, current collectors 18, and end plates 19. Hydrogen or recombinated air is supplied to an anode and oxygen or air is supplied to a cathode. These electrodes are separated by the proton exchange membrane 11. The anode undergoes oxidation, and the cathode undergoes reduction, respectively. When contacting the catalyst layer 13 (platinum or platinum alloy) proximate to the proton exchange membrane 11, hydrogen is decomposed to protons and electrons. The protons pass through the proton exchange membrane 11 to the cathode. Note that since the membrane 11 is a wet membrane, only the protons accompanying water molecules can pass through the membrane 11, while other gas cannot. The electrons move from the anode, through a device 16 and a bridge, to the cathode. The bridge is coupled between the cathode and the anode, and connected in series to the device 16. Oxygen is reduced by the electrons to oxide ions, and the oxide ions combine with the protons to form water molecules. Accordingly, the described reactions are electrochemical reactions.
Electrochemical PEMFCs have several advantages such as high efficiency, low pollution, and fast response. A PEMFC can be connected in series to enhance bridge voltage, or surface area of the electrodes thereof can be increased to enhance current flow. The device 16 may operate continuously when provided with a continuous supply of hydrogen and oxygen (generally air). Excluding small power systems, a PEMFC can be assigned as a power plant, a distributed power source, or a portable power source.
Bipolar plates make up the most of the volume and weight of the PEMFC, such that research and discovery of a new bipolar plate material is important for PEMFC development. The bipolar plates have multiple functions such as: distributing reaction gas into reaction area, separating different reaction gases (for example, oxygen and hydrogen), transmitting electricity and heat, and stabilizing the thin film electrodes. Because they are located at reaction areas of the PEMFC, chemical corrosion resistance and refraction are necessary for the bipolar plates, as well as higher volume utility and lower density thereof. Thus the basic properties of the PEMFC include high conductivity, hermeticity, chemical corrosion-resistance, refraction, high mechanical strength, low surface roughness, lightweight, and thin profile. The preferred bipolar plates should be easily manufacturable under various specifications, and mass productable with low raw material and process costs. Less costly bipolar plates with better properties will make PEMFC having higher market competitiveness.
Conventional bipolar plates include dense carbon plates, carbon composite plates, and metal plates; and more preferably dense graphite material for use in PEMFC. Dense graphite is expensive, and manufacturing cost of flow trench for the bipolar plates is also expensive. To reduce cost, composite material is a mainstream choice for used in fuel cells. Composite can be modified to bipolar plates for a fuel cell by changing the material ratio along with choosing a forming method such as molding or injection. Generally, the raw materials of composite are less costly chemicals. The dense carbon plate needs extra process for flow trenches, however, the composite can be directly formed with flow trenches and decreasing the cost of manufacturing. The dense carbon plate is a porous material, and filling the pores is necessary in post process which is time consuming and increasing additional cost. Otherwise, composite material is hermetic, thereby eliminating post process filling. Of these two materials, composite is a better choice for production of bipolar plates.
TW Pat. Pub. No. 399,348 discloses a method of forming bipolar plates of a fuel cell. In TW Pat. Pub. No. 399,348, a conductive material, a resin, and a hydrophilic agent are mixed. The mixture is molded under 500-4000 psi pressure and 250-500° C. to yield bipolar plates. The resin may be thermoplastic or thermoset, and the conductive material may be graphite powder, carbon black, or carbon fiber.
U.S. Pat. No. 6,436,315 discloses a composite bipolar plate of a fuel cell. A mixture of a resin and graphite powder is injected by a modified injection method, and various additives are classified and defined in patent '315.
U.S. Pat. No. 6,248,467 discloses a composite bipolar plate of a fuel cell. In patent '467, a vinyl ester resin and graphite powder are mixed, wherein the graphite powder has a particle size from 80 to 325 mesh.
U.S. Pat. Pub. No. 2005/0001352 A1 discloses a composite bipolar plate of a fuel cell. In U.S. Pat. Pub. No. 2005/0001352 A1, a vinyl ester resin and graphite powder are mixed, wherein the graphite powder has a particle size from 10 to 80 mesh.
Because the multilayer structure of the proton transfer film, the gas diffusion layer, and catalyst layer interposed between the bipolar plates, the bipolar plates receive huge flexural strain. The conventional bipolar plate without enough flexural strength is an obstacle to be overcome, and a method for improving bipolar plate flexural strength is desirable.